1. Field of the Invention
This invention relates to a powerful fiber-laser system having a wavelength-selective filtering unit for filtering undesirable Raman wavelengths which are reflected from internal and external obstacles along a waveguide.
2. Prior Art Discussion
Among numerous applications of powerful laser systems, metal welding and cutting occupies a prominent place. The shipbuilding and car manufacturing industries, which have been employing laser systems for processing metals for quite awhile, favor the laser systems for their efficiency and precision. However, many of the known laser systems used for welding and cutting metals suffer from low reliability. It is not unusual that in a powerful multi-cascaded laser system, at least a few and sometimes all fiber blocks and upstream terminal portion are destroyed during processing the metals.
At least one of the reasons causing a powerful laser system to malfunction during the processing of metals was never a mystery. Once a powerful light, propagating along a waveguide at the desired wavelength, hits an internal interface, such as a splice between fibers, or an external interface, such as a piece of metal, it reflects back and, under certain circumstances, may be launched into the core of a delivery fiber. As the backreflected light is guided backwards along a waveguide through fiber amplifier cascades of the system, it may become sufficiently strong to destroy fiber components. At least one condition should be met so as to produce such an unfortunate result. The power of reflected light is often substantially equal to the power of the incident light.
However, in practice, the power of reflected light propagating at the desired wavelength via, for example, an output amplifying cascade, is typically lower than that one of the forward light because at least part of the direct light signal is scattered upon heating the interface. Furthermore, typically fiber amplifiers operate in a saturated regime preventing uncontrollable increase of the power of the backreflected light. Hence, the power of the backreflected signal alone may not be sufficient to destroy components of a laser system.
Realization that the obvious cause is not solely responsible for the laser system's short life left the specialists in the laser and welding fields puzzled. The extensive research finally produced tangible results showing that, at least partially, nonlinear effects associated with powerful laser systems may detrimentally affect the reliability of these systems. The above-identified problem will become readily apparent from the following description.
FIG. 1 shows a powerful laser system 10 including an optical waveguide, typically, silica-based optical fiber laser system which has a multiplicity of alternating passive 12 and active 14 fibers spliced together. The high-and low-reflecting Fiber Bragg Gratings (FBG) 16, 18, respectively, define an optical cavity which receives upstream active fiber 14 and input and output passive fibers 12 spliced to the opposite ends of active 14. The combination of active fiber 14 and passive fiber 12, all located in the cavity, constitutes an oscillator further referred to as a master module 20. The FBGs 16 and 18 are, preferably, but not necessarily, written in respective passive fibers 12 to provide radiation at the desired wavelength. If, for example, master module 20 is configured with an Yb-doped core, the main signal can be lased, for example, at a 1070 nm wavelength. Additional cascades, as for example, an amplifier 25 may be located upstream from output amplifier 30, wherein each amplifier cascade includes a combination of active and passive fibers 14, 12 respectively. The master and amplifier modules 20, 25 and 30 are pumped by respective pump assemblies 22 and 32 operative to launch light in co-propagating, counter-propagating or opposite directions. A single or multiple filters and isolators, not shown but known to one of ordinary skills in the art are operative to attenuate reflected light signal Ir propagating along the waveguide in a reverse direction Dr.
The reflected signal Ir may be propagating in reverse direction Dr upon encountering an internal interface, such as a splice 36, and/or an external interface, for example, a surface 34 of metal to be processed. As reflected light signal Ir propagates in direction Dr at the wavelength of main signal Ip, it is first amplified by downstream amplifier 30 and further by any intermediary amplifier. Upon reaching master module 20, the power of the reflected signal reaches a level capable of destroying all of the components of the waveguide which are located upstream from amplifier module 30 if considered in the direct propagation direction. As mentioned above, however, for a reflected signal to be launched into the waveguide, it has to be mirror-reflected within a micro-or millisecond time period while metal to be processed is not melted yet. Even if this condition is met, an isolator or isolators located along the waveguide are operative to diminish the power of reflected signal to the safe levels.
Referring to FIG. 2 illustrating a spectrum of light emitted by, for example, amplifier module 30, the first power peak represents the main output signal Ip at about 1070 nm wavelength. However, the main signal is not the only one lased by amplifier module 30. Any powerful fiber laser system including continuous wave and pulsed laser systems is always associated with multiple non-linear effects detrimentally affecting the efficiency of these systems. One of the non-linear effects is stimulated Raman scattering—an optical process that involves light radiation at a wavelength(s) longer than the main light signal. In the illustrated spectrum of FIG. 2, one or more weak Raman components Ira 50 of main signal Ip are generated in each of multiple cascades including amplifier module 30 at longer than the desired wavelength(s) and propagate towards the internal or external interface 36 and 34, respectively, of FIG. 1. The power level of first Raman component Ira (let alone successive Raman components) is much less than that one of the main signal. For instance, the main signal may easily reach a level of about 80-100 dB, while the Stokes component may be as weak as about −20 dB at the output of system 10. Seemingly, the latter with such a negligible level of power, when reflected from an interface, cannot jeopardize the safety of system 10. In practice, amplifying module 30, typically working in a saturated regime, greatly amplifies weak signals, such as the reflected Raman component, while the strong reflected main signal Ir is only slightly amplified. Due to considerable lengths of system 10, a backreflected Raman component, further referred to as a backreflected Raman signal, may be devastating for system 10 when coupled to the reflected main signal Ir. In fact, as numerous experiments show, reflected Raman signals may reach up to 60 and even higher dB at the point when it reaches, for example, master module 20 of FIG. 1. Such a powerful signal may be sufficient to destroy fiber components along the upstream stretch of system 10.
A need, therefore, exist for a method of preventing distortion of powerful fiber laser systems by backreflected parasitic signals reflected from internal and external obstacles.
A further need exists for a powerful laser system capable of preventing propagation of parasitic back-reflected signals along the system.